This invention relates to an improved buried-channel MOS transistor and a process of producing the improved transistor.
Conventional buried p-channel MOS transistors have a shallow p-type diffused layer in an n-type substrate surface under the gate electrode and gate oxide film in order to adequately lower the threshold voltage. The shallow p-type diffused layer provides a channel between source and drain regions which are more heavily doped p-type diffused layers.
An advantage of buried-channel MOS transistors over surface channel MOS transistors is increased mobility of electric charge because of little scattering at the interface between the substrate and the gate oxide film. On the other hand, a disadvantage of buried-channel MOS transistors is susceptibility to the so-called short channel effect. That is, since the source and drain regions and the channel region are of the same type conductivity, the application of a voltage to the drain causes wider expansion of the depletion layer than in surface channel MOS transistors and hence is liable to result in a phenomenon called punch-through, i.e. flow of current between source and drain as a result of the drain voltage directly forcing down the potential energy at the boundary between source and channel. In this case the punch-through occurs in the vicinity of the substrate surface and hence is called surface punch-through. The fundamental reason for the liability to punch-through is that effective channel length becomes considerably shorter than the physical length of the gate electrode. Therefore, it is difficult to further miniaturize buried-channel MOS transistors by shortening the physical length of the gate electrode.